Valence fluctuations trigger superconductivity in a plutonium compound

  • Brad Ramshaw
  • Ross David Mcdonald
  • Jonathan Bobby Betts
  • Charles H Mielke
  • Eric Dietzgen Bauer
  • Mitchell, Jeremy Neil
  • Paul H Tobash
  • Albert Migliori

Press/Media: STE Highlight

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Los Alamos researchers have pinpointed the mechanism behind superconductivity in plutoniumcobalt- gallium5 (PuCoGa5), a compound with a surprisingly high superconducting transitiontemperature. The team leveraged the power of resonant ultrasound spectroscopy (RUS) to probe a sample from room temperature to 10 degrees above absolute zero (Kelvin). This discovery, in a heavy-Fermion compound first identified at the Lab more than a decade ago, helps scientists understand which material properties lead to high superconducting transition temperatures. Their ultimate goal is to design a room-temperature superconductor. Room-temperature superconductors are of interest from a fundamental scientific perspective as well as for a variety of applications including superconducting magnets for cheaper and more powerful magnetic resonance imaging (MRI).

 

In ordinary members of the heavy-fermion superconductor family, scientists seek new

superconductors by suppressing magnetism and activating superconductivity through spinfluctuations. However, PuCoGa5 does not have the required magnetism to support this mechanism of superconductivity. According to Los Alamos research published in the Proceedings of the National Academy of Sciences , plutonium valence fluctuations are key to understanding the source of high-temperature superconductivity in PuCoGa5, the highest Tc superconductor of the heavy fermions, with a Tc of 18.5 K. (Tc refers to the critical temperature below which a superconductor must cool in order to exhibit this unusual state of matter in which electrical current flows without resistance as a result of the material’s electrons acting in pairs.) A critical component of the discovery was the observation that the valence fluctuations disappear at Tc. This finding suggests that the same electrons that participate in the valence fluctuations also participate in superconductivity, providing a direct link between the two phenomena.

 

Elastic moduli measurements are a powerful tool for revealing valence fluctuations. Therefore, the Los Alamos researchers improved upon the sensitivity of resonant ultrasound spectroscopy (RUS) to resolve all the elastic

moduli of PuCoGa5 to low temperature in a way that was impossible with more traditional ultrasound techniques. Resonant ultrasound spectroscopy provides the highest absolute accuracy of any routine elastic modulus measurement technique, and it does this quickly on small samples. The National High Magnetic Field Laboratory (NHMFL) at LANL provides a RUS user facility for the general science community. The facility’s capabilities range from 300 mK to 600 K and to 20 T. This provided a unique opportunity to explore the unusual valence of plutonium with a thermodynamic, symmetry-sensitive probe, allowing the scientists to find evidence for fluctuations of the plutonium 5f mixed-valence state.

 

The researchers found that that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability. Therefore, the team identified the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc. This result suggests that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity appears to be mediated by magnetic fluctuations. The new observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.

 

Reference: “Avoided Valence Transition in a Plutonium Superconductor,” Proceedings of the National Academy of Sciences 112, 3285 (2015); published online before print March 3, 2015, doi: 10.1073/pnas.1421174112. Authors: Brad Ramshaw, Ross McDonald, Jon Betts, Chuck Mielke, and Eric Bauer (Condensed Matter and Magnet Science, MPA-CMMS); Arkady Shekhter (MPACMMS, now with NHMFL-Tallahassee); Jeremy Mitchell and Paul Tobash (Nuclear Materials Science, MST-16); and Albert Migliori (National Security Education Center, NSEC).

 

Work at Los Alamos National Laboratory was performed under the auspices of the DOE, Basic Energy Sciences, Division of Materials Sciences and Engineering, and the LANL Laboratory Directed Research and Development (LDRD) Program. This work was conducted at the National High Magnetic Field Laboratory, which the National Science Foundation and the State of Florida fund. The research supports the Lab’s Energy Security mission area and Materials for the Future science pillar through the development of materials for energy applications. Technical contact: Brad Ramshaw

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Image 1

 

Caption 1

Photo. The Lab team used this probe to locate the mechanism behind superconductivity in plutonium-­‐cobalt-­‐gallium5.

 

Image 2

Caption 2

 

Figure 4. The temperature-­‐dependent elastic moduli of PuCoGa5. The six elastic moduli in PuCoGa5 can be grouped into two categories: shear moduli (left panel), which characterize how easily the material is distorted in a volume-­‐preserving way (like stretching it along its diagonal); and compressional moduli (right panel), which characterize how easy it is to change the volume of the material. The shear moduli all show conventional behavior for a metal, becoming stiffer at low temperature. However, the compressional moduli stiffen to about 50 degrees above absolute zero, and then start to soften again. This unexpected behavior indicates valence fluctuations.

PeriodApr 29 2015

Media coverage

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Media coverage

  • TitleValence fluctuations trigger superconductivity in a plutonium compound
    Date04/29/15
    PersonsBrad Ramshaw, Ross David Mcdonald, Jonathan Bobby Betts, Charles H Mielke, Eric Dietzgen Bauer, Jeremy Neil Mitchell, Paul H Tobash, Albert Migliori, Brad Ramshaw, Ross David Mcdonald, Jonathan Bobby Betts, Charles H Mielke, Eric Dietzgen Bauer, Paul H Tobash, Albert Migliori

Media Type

  • STE Highlight

Keywords

  • LALP 15-001

STE Mission

  • Energy Security

STE Pillar

  • Materials for the Future

STE Publication Year

  • 2015